PROJECT SUMMARY The major goal of this proposal is to define critical cerebrovasculature pathways mediating the clearance of amyloid-? (A?), the aggregation of which into amyloid plaques represents a pathological hallmark of Alzheimer's disease (AD). In so doing, we will evaluate the specific role of LDL receptor-related protein 1 (LRP1), heparan sulfate proteoglycan (HSPG) and apolipoprotein E (apoE) isoforms in cerebrovascular clearance of A? and formation of cerebral amyloid angiopathy (CAA). The aggregation of A? in the brain is a direct result of its increased brain concentration due to an imbalance of its production and clearance. Although brain A? clearance is mediated by multiple pathways including intracellular degradation and extracellular degradation, much remains unknown about how the cerebrovasculature system clears A? through local cellular, blood-brain barrier (BBB), and perivascular drainage pathways. Given impaired clearance of A? drives late-onset AD (LOAD), we aim to improve understanding of the pathways regulating A? clearance, thereby establishing new targets for AD therapy and prevention that will benefit the vast majority of patients. During the previous funding cycle, we employed several conditional mouse models to demonstrate that deletion of the A? receptor LRP1 leads to slower A? clearance and exacerbated amyloid pathology, while deletion of another A? receptor HSPG in neurons produces the opposite effects. In addition, we and others have shown that apoE, a ligand for LRP1 and HSPG, modulates A? metabolism and pathology in an isoform- dependent manner with apoE4, whose gene allele represents the strongest genetic risk for AD, promoting amyloid deposition and the formation of CAA. Thus, the overall goal of this renewal application is to define the molecular mechanism underlying cerebrovascular clearance of A?. We hypothesize that the A? receptor LRP1 promotes, whereas HSPG inhibits, A? clearance along the cerebrovasculature in an apoE isoform-dependent manner impacting the formation of CAA and the distribution of A? pathology. In Aim 1, we will define the roles of LRP1 and HSPG in cerebrovascular function, clearance of A?, and formation of amyloid plaques and CAA using conditional mouse models inducing vasculature deletion of A? receptors at different ages and at different stages of plaque/CAA pathology. In Aim 2, we will analyze how apoE isoforms affect A? clearance and pathology in cerebrovasculature using cell type-specific and inducible mouse models. In Aim 3, we will define the molecular mechanisms through which LRP1, HSPG and apoE isoforms modulate A? metabolism, BBB integrity and vascular structure using reconstructed model systems from primary mouse cells or induced pluripotent stem cell (iPSC)-derived human cells to improve the likelihood of discoveries translatable to human AD. Finally, we plan to perform unbiased, single cell-type transcriptome analysis to uncover signaling pathways downstream of LRP1/HSPG/apoE. Together, our studies will define the molecular mechanism(s) underlying brain A? clearance and establish new targets for mechanism-based therapy.